1. Field of the Invention
The invention relates generally to the field of electric motors having brakes.
2. Description of Related Art
Many applications that require electric motors also require brakes, hence there are a multitude of inventions combining these two technologies. In particular, electrically motorized wheel hub applications often require a mechanism for slowing the wheel, or holding the wheel stationary.
In one example, U.S. Pat. No. 5,438,228 to Couture et al. describes an electrically motorized wheel assembly where the electric motor has an outer-rotor configuration, and a mechanical brake assembly includes a disc located adjacent to the stator and having a diameter smaller than an inner diameter of the rotor.
In another example, U.S. Pat. No. 5,465,802 to Yang discloses an electric motor located within a wheel and having means for disengaging the motor from the wheel axle. In particular, a rotor is fixed inside a wheel frame, and a stator is located within the rotor and is attached to an axle. A brake element fixes the stator relative to the axle when actuated, so that the stator and the axle rotate together. When the brake element is not actuated, the stator rotates freely about the axle with the wheel when no current is supplied to the motor. The brake does not brake the motor output, but instead engages and disengages the motor with respect to the wheel axle, thus allowing the wheel to rotate freely when no current is supplied to the motor.
U.S. Pat. No. 3,626,220 to Niculescu, U.S. Pat. No. 4,022,301 to Hansen, U.S. Pat. No. 4,103,763 to Glockner et al., U.S. Pat. No. 4,494,058 to Berti, No. 4,910,423 to Werber, U.S. Pat. No. 5,050,711 to Sawato, U.S. Pat. No. 5,121,018 to Oldakowski and U.S. Pat. Nos. 5,306,989 and 5,406,180 to Feller describe various electric motors each having an inner-rotor configuration and a brake mechanism that is larger in diameter than the motor rotor.
Such motor and brake combinations can be relatively bulky. They are also suboptimal for some applications, such as wheel drives, where it desirable to minimize an axial length of the electric motor and brake components.
Alternatively, electric current can be applied to the motor in a controlled fashion to generate a force between the rotor and the stator that maintains the position of the rotor relative to the stator and thus holds the wheel stationary. If the wheel is moving, the current can be controlled to generate a force between the rotor and the stator that reduces the velocity of the rotor relative to the stator, thus slowing the wheel.
However, it may not be desirable to expend electrical energy to hold the wheel parked and stationary. In addition, mechanical brakes are typically simpler, less expensive, more reliable and consume less electrical energy (if any) than electric motor systems that generate braking force by selectively applying electrical power to the motor.